1. Field of the Invention
The present invention relates to an optical pickup system, and more particularly to an optical pickup system capable of recording/reading out information on/from mini discs or magneto-optical discs.
2. Description of the Prior Art
FIG. 1 illustrates a construction of a conventional optical pickup system.
The optical pickup system shown in FIG. 1 for recording optical information on a disc 21 or reading out the optical information from the disc 21 includes a laser diode 11 which is a light source, a grating 12, a collimator 13, a polarizing beam splitter (hereinafter simply referred to as "PBS") 14, a reflection mirror 15, an objective lens 16, a modified Wollaston prism 17, an image-forming lens 18, a concave lens 19 and a photodetector divided-by-eight 20.
The grating 12 allows laser beams from the laser diode 11 to be one main beam for reading out the optical information recorded on the disc 21 and detecting a focus error and to be two sub-beams for detecting a tracking error of the disc 21. The collimator 13 makes the one main beam and two sub-beams passed through the grating 12 be parallel beams.
The PBS 14 fully reflects 100% of the S-wave component of the beams incident from the collimator 13 while reflecting some P-wave component and transmitting the other P-wave component to the reflection mirror 15. Also, the S-wave component of the beams reflected from the disc 21 is totally reflected to the Wollaston prism 17, and the P-wave component is partially reflected to the Wollaston prism 17 and partially transmitted.
The reflection mirror 15 reflects the three beams having P-wave component passed through the PBS 14 toward the disc 21, or reflects the three beams having P-wave and S-wave components reflected from the disc 21 to the PBS 14.
The objective lens 16 focuses the three beams having P-wave component reflected via the reflection mirror 15 onto the disc 21, or again alters the three beams having mixed P-wave and S-wave component reflected from the disc 21 to be parallel beams.
The Wollaston prism 17 receives the three beams reflected from the disc 21 via the PBS 14. In addition, as shown in FIG. 4, the main beam is separated into beams of S-wave, P-wave and (P+S)-wave components and the sub-beams are separated into beams of P-wave and S-wave components. Then, five beams of the separated three main beams and two sub-beams are incident to the image-forming lens 18.
The image-forming lens 18 is for concentrating five beams passed through the Wollaston prism 17. The concave lens 19 having a toric surface increases the angle of the beam passed through the image-forming lens 18 and, at the same time, produces astigmatism to the main beam passed through the image-forming lens 18 to detect the focus error.
The photodetector 20 divided-by-eight is partitioned into eight split areas as shown in FIG. 2, in which the central split areas a, b, c and d of the eight split areas are used for being focused by the beam of (S+P)-wave component separated from the main beam incident from the concave lens 19, so that the focus error is detected by a signal detected in the areas a, b, c and d. The areas e and f on the upper and lower portions of the areas a, b, c and d are for being focused by the beams of the P-wave and S-wave components respectively separated from the sub-beams, so that the tracking error is monitored by a difference between signals detected in the areas e and f. The areas i and j on the right and left of the areas a, b, c and d are for being focused by the beams of S-wave and P-wave components separated from the main beam, so that the optical information recorded on the disc 21 is read out by a signal detected on the areas i and j.
The operation of the conventional optical pickup system having the above-mentioned construction will be described with reference to FIGS. 2 to 6 as below.
The laser beams from the laser diode 11 which is the light source are diffracted into a main beam L1 and two sub-beams L2 and L3. The three beams L1 to L3 of the main beam and sub-beams are altered into the parallel beams by the collimator 13 to be incident to the PBS 14.
The PBS 14 reflects 100% of the S-wave component of the three beams and half of the P-wave component. The remaining 50% of the P-wave component are is transmitted. Therefore, the S-wave components of the incident beams are totally reflected by the PBS 14, and the P-wave component is partially reflected and partially transmitted by the PBS 14 to be incident to the reflection mirror 15. The reflection mirror 15 receives the beams of P-wave component from the PBS 14 to reflect the incident beams toward the disc 21, and the reflected beams focus onto the disc 21 via the objective lens 16.
As illustrated in FIG. 3, the three beams L1 to L3 of one main beam and two sub-beams focus on tracks 21-1 of the disc 21. Among the three beams, the main beam L1 is used for reading out the information and detecting the focus error, and two sub-beams L2 and L3 are used for detecting the tracking error. The three beams focusing on the tracks 21-1 of the disc 21 are reflected from the disc 21 while containing information required for reading out the optical information recorded on the disc 21 and detecting the focus error and tracking error. Here, the recorded optical information denotes pit information or kerr rotation by the magnetization direction.
At this time, the beams of P-wave component focus onto the disc 21, but the property differs in accordance with the presence and absence of the information on the tracks 21-1 of the disc 21. In more detail, if the information is not recorded on the track 21-1 of the disc 21, the beam reflected from the disc has the P-wave component without including the S-wave component, but the beam from the disc 21 is the mixed beam having both S-wave and P-wave components when the track 21-1 of the disc 21 has the information thereon.
The three beams reflected from the disc 21 via the objective lens 16 are incident to the PBS 14 by means of the reflection mirror 16. The PBS 14 reflects all S-wave component of the incident beams to the Wollaston prism 17, reflects 50% of P-wave component to the Wollaston prism 17, and transmits 50% of P-wave component. Accordingly, all S-wave component of the beams reflected from the disc 21 are reflected by the PBS 14 to be incident to the Wollaston prism 17, and only half of P-wave component is reflected by the PBS 14 to be incident to the Wollaston prism 17.
As shown in FIG. 4, the Wollaston prism 17 separates the incident main beam into three beams of S-wave, P-wave and (S+P)-wave components. Also, the sub-beams are separated into two beams of P-wave and S-wave components. Thereafter, the three beams reflected from the disc 21 are separated into the five beams via the Wollaston prism 17 to be incident to the concave lens 19 via the image-forming lens 18.
The concave lens 19 which has the toric surface for producing the astigmatism increases the angles between respective five beams incident from the image-forming lens 18 and, simultaneously produces the astigmatism with respect to the main beam for detecting the focus error. The five beams passed through the concave lens 19 focus onto the photodetector 20 divided-by-eight as shown in FIG. 5.
Therefore, in accordance with the shapes of the five beams focusing on respective areas of the photodetector 20 divided-by-eight, the tracking error and focus error are detected, and the information recorded on the optical disc 21 is read out.
To begin with, a tracking error signal TES by means of the sub-beams is detected by the beams focusing on the split areas e and f of the photodetector 20 divided-by-eight, which is given by the following equation (1). EQU TES=Se-Sf (1)
where reference symbols Se and Sf respectively denote electrical signals of the beams focusing on the split areas e and f of the photodetector 20 divided-by-eight.
On the other hand, a focus error signal FES is detected by the beams focusing on the split areas a, b, c and d, which is expressed as: EQU FES=(Sa+Sc)-(Sb+Sd) (2)
where reference symbols Sa, Sb, Sc and Sd respectively denote electrical signals of the beams focusing on the split areas a, b, c and d of the photodetector 20 divided-by-eight.
FIG. 6 illustrates the pattern variation of the beams focusing on the split areas a, b, c and d of the photodetector 20 divided-by-eight, in which the shape of the beam focusing on each split area varies in accordance with the change of the distance between the disc 21 and objective lens 16. That is, respective focusing patterns of the beams onto the split areas a, b, c and d are illustrated that no focus error appears by the normal spacing of the objective lens 16 from the disc 21, as shown in FIG. 6A, but the focus error appears due to a remote distance between the objective lens 16 and disc 21, as shown in FIG. 6B or a close distance between the objective lens 16 and disc 21 as shown in FIG. 6C.
In case of a magneto-optical disc, information of grooves formed in the disc is detected by constituting a signal system such as:
Address in Pregroove (ADIP)=(Sa+Sd)-(Sb+Sc) or Absolute Time in Pregroove (ATIP).
As can be noted in the above equations (1) and (2), the tracking error signal TES becomes zero and the focus error signal FES equals zero when neither the tracking error nor the focus error occur.
The information recorded on the disc 21 is read out by means of the main beam of S-wave component focusing on the split area i of the photodetector 20 divided-by-eight and the main beam of P-wave focusing on the split area j thereof.
When a magneto-optical signal (kerr rotation by the magnetization direction) is read out, the optical information is read out by a signal difference of the beams focusing on the split areas i and j as defined by: EQU Optical Information Signal (magneto-optical signal)=Si-Sj (3)
Meanwhile, a pit signal having an uneven shape recorded on the disc 21 is read out by variation of the amount of the beams focusing on the split areas i and j of the photodetector 20 divided-by-eight as shown in the following equation: EQU optical information signal (pit signal)=Si+Sj
where the reference symbols Si and Sj respectively denote electrical signals of the beams focusing on the split areas i and j of the photodetector 20 divided-by-eight.
However, the conventional optical pickup system as described above has drawbacks. More specifically, since the astigmatism has heretofore used for detecting a focus error by means of three beams, a concave lens having the toric surface which is expensive and is difficult in fabricate is employed to generate the astigmatism. Moreover, a modified Wollaston prism involving a fastidious fabrication process is utilized to separate a mixed beam of (P+S)-wave component for detecting the focus error from a main beam reflected from a disc. In order to allow beams from a laser diode to be partially incident to the disc or beams reflected from the disc to be partially incident to the modified Wollaston prism, a pentagonal polarized beam splitter is applied which is also difficult to be fabricated.
As the result, the conventional optical pickup system involves problems of complicated structure and expensive manufacturing cost since numerous optical elements are used for reading out information recorded on the disc which includes expensive elements which are difficult to fabricating. Furthermore, the increased number of optical elements raises the inherent weight of the optical pickup system to lengthen access time for reading out the information recorded on the disc, thereby delaying the speed of reading out the information.